The present invention relates to an imaging system using coherent light radiation to expose a layered member in an image configuration and, more particularly, to a method for forming the imaging member so as to reduce optical interference occurring within said member which results in a plywooding type of defect in output prints.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser, is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered photoreceptors" have at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon the physical characteristics, is an interference effectively created by two dominant reflections of the incident coherent light on the surface of the photoreceptor; e.g. a first reflection from the top surface and a second reflection from the bottom surface of the relatively opaque conductive ground plane. This condition is shown in FIG. 1: a coherent beam is incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. The interference effects can be explained by following two typical rays of the incident illumination. The two dominant reflections of a typical ray 1, are from the top surface of layer 7, ray A, and from the top surface of ground plane 9, ray C. The transmitted portion of ray C, ray E, combines with the reflected portion of ray 2, ray F, to form ray 3. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, the interference of rays E and F can be constructive or destructive when they combine to form ray 3. The transmitted portion of ray 2, ray G, combines with the reflected portion of ray C, ray D, and the interference of these two rays determines the light energy delivered to the generator layer 8. When the thickness is such that rays E and F undergo constructive interference, more light is reflected from the surface than average, and there will be destructive interference between rays D and G, delivering less light to generator layer 8 than the average illumination. when the transport layer 7 thickness is such that reflection is a minimum, the transmission into layer 8 will be a maximum. The thickness of practical transport layers varies by several wavelengths of light so that all possible interference conditions exist within a square inch of surface. This spatial variation in transmission of the top transparent layer 7 is equivalent to a spatial exposure variation of generator layer 8. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. The output copy exhibits a pattern of light and dark interference fringes which look like the grains on a sheet of plywood, hence the term "plywood effect" is generally applied to this problem.
In the prior art, various techniques are known for modifying the structure of the imaging member to reduce the second dominant reflection from the imaging member ground plane. U.S. Pat. No. 4,618,552 and copending application, U.S. Ser. No. 07/546,990, filed on Jul. 2, 1990 now U.S. Pat. No. 5,096,792, describe methods of roughening the surface of the ground plane to create a diffuse reflection of the light reflected therefrom. U.S. Ser. No. 07/541,655, filed on Jun. 21, 1990, now abandoned, discloses a roughening of the PET substrate upon which the ground plane is formed with the roughening replicated into the ground plane. U.S. Ser. No. 07/523,639, filed on May 15, 1990, now U.S. Pat. No. 5,051,328, and U.S. Ser. No. 07/552,200, filed on Jul. 13, 1990, now U.S. Pat. No. 5,139,907 disclose forming the ground plane or a layer over the ground plane, respectively, of a transparent conductive material. U.S. Ser. No. 07/646,117, filed on Jan. 28, 1991, now U.S. Pat. No. 5,069,758, discloses an electroforming process for the imaging member ground plane, which results in a ground plane with a smooth, dull surface.
The present invention is directed towards eliminating the reflections from the ground plane by forming the imaging member with a conductive ground plane with a black nickel surface. The black surface absorbs, rather than reflects, the incident light. Since the light absorbs, secondary reflections which create cross-talk among pixels at the member surface are eliminated. This eliminates a problem which was present in prior art systems which taught methods of diffusely reflecting light from the ground plane surface. Also present in the diffusely reflecting prior art concepts, for each wavelength of incident light, there was an optimum roughness to the diffusely reflecting surface. The black belt of the present invention absorbs all wavelengths; hence enabling a wider manufacturing latitude. More particularly, the present invention is directed towards an improved photosensitive imaging member having at least a conductive ground plane with an overlying charge transport and charge generator layers, the improvement wherein said ground plane has a smooth black finish which absorbs all wavelengths of light incident thereon.